1. Field of the Invention
The present invention relates to a method for continuous casting of molten steel and apparatus therefor, and more particularly to a method for continuously casting molten steel by electromagnetically stirring molten steel and apparatus therefor.
2. Description of the Related Arts
As a method for minimizing center segregations of solidification structure by increasing fine equi-axed crystals, there are pointed out a low-temperature casting method and an electromagnatic stirring method. In the low-temperature casting method, inhomogeneous nuclei are easily produced by making superheating of the molten steel as small as possible during casting of liquid metal. This method wherein fine equi-axed crystals can be obtained is known as the simplest method for improving the solidification structure.
In the elecromagnetic stirring method, the equi-axed crystal structure is obtained by dividing dendrite arms by forcedly flowing molten steel adjacent to a solidification interface. As the electromagnetic stirring method, there are pointed out a linear motor type, rotary type and magnetostatic field type electromagnetic stirring methods. In the linear motor type and rotary type electromagnetic stirring methods, a shifting magnetic field is applied to molten steel, and the molten steel is forcedly flowed by an interaction of an eddy current generated in the molten steel with the applied magnetic field. In the magnetostatic electric field type electromagnetic stirring method, Lorentz's force is obtained by constantly feeding electric current to molten steel, to which a static magnetic field is applied.
FIG. 7 is an explanatory view showing a situation wherein molten steel adjacent to a meniscus inside a continuous casting mold is stirred by the rotary type electromagnetic stirring apparatus along the inner circumferential surfaces of the mold.
An electromagnetic stirring coil 22 surrounding the continuous casting mold 21 is positioned at a level of a height containing the meniscus of the molten steel outside the continuous casting mold 21. The molten steel is stirred by generating a rotating magnetic field inside the mold by means of the electromagnetic coil 22. Dendrite arms generated along the inner circumferential surfaces of the mold 21 are divided by this stirring whereby an equi-axed crystal structure is obtained.
An electromagnetic stirring force has to be increased to increase the ratio of equi-axed crystals. Since molten steel adjacent to the inner circumference of the mold is raised by a centrifugal force as shown in FIG. 7 when the electromagnetic stirring force is increased, the thickness of a powder pool 24 of lubricating powder on the molten steel 23 inside the continuous casting mold 21 becomes small. Unmelted powder is entrapped into the molten steel whereby slag spots are generated. As the result that powder flows non-uniformly into between the mold 21 and a solidified shell since air is included into the powder, the powder pool being flowed, the rate of solidification of the molten steel becomes small in parts. In consequence, longitudinal cracks are generated on the surface of a billet.
Further, when a flow of molten steel is generated in front of the solidified shell by the use of an electromagnetic stirring apparatus, a negative segregation zone is generated since a concentrated molten steel among the dendrites is washed.
Japanese Patent Publication Laid Open No. 70361/89 discloses a method wherein an electromagnetic coil is arranged in an outer circumference of a continuous casting mold and a round electrically conductive ring is arranged adjacent to a meniscus of the molten metal to apply perpendicularly and upwardly a magnetic field to molten metal. However, this method does not relate to the rotating elecromagnetic stirring method.